1. Field of the Invention
The present invention relates to new polyaddition compounds containing uretdione groups, to a process for their preparation and to their use as a starting component for the preparation of polyurethane plastics, in particular as a crosslinking agent for powder coatings that can be crosslinked by heat.
2. Description of the Prior Art
Polyaddition compounds containing uretdione groups are increasingly used as crosslinking agents, which are free from blocking groups for polyurethane (PU) powder coatings having high resistance to weathering. Thermal cleavage of the uretdione groups back into free isocyanate groups and subsequent reaction thereof with a hydroxy-functional binder is utilized as the crosslinking principle with these compounds.
Although uretdione powder coating crosslinking agents are known they have not been able to find acceptance in the market. The reason for this is, inter alia, on the one hand a comparatively low isocyanate content of the products currently commercially available, and on the other hand a low average functionality, which is frequently less than 2.0. However, crosslinking agent components having a high NCO content and the highest possible functionality, which is at least 2.0, are desirable to provide an adequate crosslinking density, which in paints and coatings is equivalent to good resistance properties.
EP-A 45 994 describes reaction products of a linear, difunctional IPDI-uretdione and less than stoichiometric amounts of simple diols, which optionally contain ether groups, as powder coating crosslinking agents which are free from blocking agents. These polyaddition products, which may contain up to 8 wt. % of non-blocked free terminal isocyanate groups, are indeed distinguished by high total isocyanate contents (total of isocyanate groups present in free form and in the internally blocked form as uretdione groups) and a functionality of 2.0, but their preparation is difficult, especially on an industrial scale.
Since cleavage of the thermolabile uretdione groups back into the free form noticeably begins at 110.degree. C. in the presence of hydroxy-functional reaction partners, this temperature should not be exceeded in the synthesis of polyaddition compounds containing uretdione groups. Nevertheless, in this temperature range the products described in EP-A 45 994 have such high melt viscosities that their capacity for production in the melt is limited on an industrial scale because of inadequate stirrability. On a large industrial scale, such uretdione crosslinking agents can be prepared only as a solution in solvents which are inert towards isocyanates. The auxiliary solvent must then be separated off in an additional process step. A solvent free preparation process for such polyaddition compounds, which is described in EP-A 669 354 and in which the starting materials can be reacted with one another in special reaction screws or extruders under the action of high shear forces, requires a very high outlay and is very expensive in terms of apparatus.
The main disadvantage of the polyaddition products described in EP-A 45 994, however, is the relatively high content of non-blocked free isocyanate groups which react with the hydroxy-functional binder during the preparation of the powder coating, usually by melt extrusion at approx. 100.degree. C. Powder coatings pre-crosslinked in this way show such unfavorable rheological properties in the stoving process that the resulting coats have only moderate optical properties after curing, in particular a completely inadequate flow.
The problem of undesirable pre-crosslinking exists in principle for all PU powder coating hardeners, which have a high content of free isocyanate groups. For example, in the case of the powder coating crosslinking agents described in EP-A 45 996 and EP-A 45 998, which are free from blocking agents and are obtained from linear IPDI-uretdione and simple diols, the terminally free isocyanate groups are at least partly reacted with monoamines or monoalcohols. The troublesome preliminary reaction in the extruder can indeed be suppressed in this manner, but the chain termination with the monofunctional compound significantly reduces the total isocyanate content, and at the same time the average functionality falls to below 2.0. Also, the products of these two publications have such high melt viscosities at the cleavage temperature of the uretdione groups back into the free form, that they can be prepared reliably and reproducibly only in solution or by the expensive process of EP-A 669 354.
EP-A 639 598 relates to the preparation of uretdione powder coating crosslinking agents using chain lengthening agents containing ester groups or carbonate groups. The products obtained by this process have such low melt viscosities at temperatures only slightly above their melting point that a solvent free production is possible for the first time in a simple stirred apparatus. The uretdione crosslinking agents described in EP-A 720 994, in which dimer diols are employed for chain lengthening, have melt viscosities which are sufficiently low for a solvent free preparation. However, because of the relatively high molecular weight of the chain lengthening agents employed, the products of both publications have reduced total isocyanate contents compared with those of EP-A 45 996.
According to EP-A 669 353, polyurethanes containing uretdione groups, terminal hydroxyl groups, and an average functionality of 2.0 can be prepared in special intensive kneaders by the solvent free reaction of linear IPDI dimers with an excess of diols and/or polyester diols. The use of at least trifunctional polyols and/or branched polyester polyols allows for the preparation of OH-terminal uretdione crosslinking agents having functionalities of between 2 and 3 or .gtoreq.3, as shown in EP-A 825 214 and EP-A 780 417. However, the advantage of the high functionality in these polyaddition compounds is counteracted by the disadvantage that the hydroxyl groups present in the crosslinking agent molecule also react under stoving conditions with urethanization, and in this way significantly reduce the internally blocked NCO content available for crosslinking of the binder.
Uretdione powder coating crosslinking agents prepared from HDI dimers optionally containing isocyanurate groups are known from EP-A 760 380. Because of the uretdione groups, these products have very high latent NCO contents, but show such a low tendency towards crystallization, which is usual for HDI-polyisocyanates, that even with intensive cooling they harden completely only after hours or even days. A preparation on an industrial scale is therefore not practical.
EP-A 790 266 relates to polyaddition products which contain uretdione and isocyanurate groups and are obtained from an IPDI dimer prepared by trialkyl-phosphine catalysis, diols and/or di-secondary diamines as chain lengthening agents and optionally monoalcohols or monoamines. These products can also have relatively high contents of free isocyanate groups and, therefore, a high total isocyanate content coupled with a high functionality. Because of the branched starting polyisocyanates, which are obtained with widely varying trimer contents in each case depending on the particular dimerization and working up conditions, the preparation succeeds only after expensive determination of the precise functionality and only in organic solution.
No uretdione powder coating crosslinking agents are known which meet all the requirements in practice and in particular have high total contents of isocyanate groups coupled with the highest possible crosslinking agent functionality and at the same time may be prepared in a simple process without problems.
It is an object of the present invention to provide PU powder coating crosslinking agents, which contain uretdione groups and do not have the disadvantages described above for the prior art.
This object may be achieved with the polyaddition compounds according to the present invention described below and the process for their preparation. The polyaddition compounds according to the invention are based on the surprising observation that by reaction of linear uretdione polyisocyanates with less than stoichiometric amounts of polyols containing ester and/or carbonate groups by a solvent free process in simple apparatus, compounds are prepared that are suitable as powder coating crosslinking agents.
In combination with commercially available powder coating binders, these compounds provide coatings which are distinguished by outstanding flow and very high gloss, in spite of a high content of free isocyanate groups. This was surprising since according to the doctrine of EP-A 639 598 it was to be assumed that such polyaddition compounds containing ester and/or carbonate groups are suitable for use as powder coating crosslinking agents only when the content of free isocyanate groups is less than 2.0 wt. %. Additionally, it is also pointed out in EP-A 760 380 (p. 7, lines 34 to 40) that products having too high a content of non-blocked isocyanate groups lead to coatings with unsatisfactory optical properties because of the unavoidable pre-crosslinking in the extruder.
Although hydroxypivalic acid neopentylglycol ester is mentioned within a long series of simple diols in EP-A 669 354 as a suitable starting compound for the solvent free preparation of polyaddition compounds containing uretdione groups in intensive kneaders, the expert has not been able to obtain any concrete indication at all about the present invention from this publication. Specifically, there is no indication that linear uretdione polyisocyanates can be reacted with polyols containing ester groups and/or carbonate groups in simple stirred apparatus in the melt to give highly functional uretdione powder coating crosslinking agents, which have very high total isocyanate contents and provide paint films of outstanding quality with conventional polyester polyols, in spite of a considerable content of free isocyanate groups. Rather, it can be shown that in contrast to the process products according to the invention, uretdione powder coating crosslinking agents having free isocyanate groups prepared with the exclusive use of hydroxypivalic acid neopentylglycol ester as the chain lengthening agent containing ester groups results in coatings having considerable surface defects, in particular a pronounced orange-peel effect.